Bridge decking is relatively well known. Typically, bridge decking includes two panels or skins lying in a generally horizontal plane (or other intended plane of roadway), parallel to one another. The skins are spaced vertically apart by internal vertical, slanted, trapezoidal, or triangular members similar in nature to the way webbing spaces two horizontal surfaces in I-beams. The internal webbing serves to distribute loads imparted on the horizontal surfaces. Additionally, the webbing rigidly resists deflection and torsion resulting from the loads imparted on the horizontal panels.
Creating bridge decking from lightweight metals, such as aluminum, is advantageous for several reasons. Decreasing the weight of decking in a bridge structure may allow for increased service loads. Aluminum bridge decking can be prefabricated in modular units and joined together on site when placed in service. Aluminum bridge decking is more easily transported that heavier metals or preformed concrete. Aluminum bridge decking may be employed in new structures, or it may be used to refurbish an aging bridge structure.
Modular bridge decking members are typically joined at their vertical seam abutments by various welding, filling, or fastening methods. Welding the decking faying surfaces (abutments) typically provides for more rigidity and increased load distribution, whereas non-welded fasteners allow enhanced and semi or fully-independent movement as between modular decking members under changing load conditions. In applications where welded joints are desired, the use of friction stir welding (“FSW”) techniques has developed as one possible method for joining the members.
Friction stir welding generally includes the application of a pin or probe to the surface of a joint or seam. The pin applies pressure and fiction, typically by spinning, on the seam sufficient to cause the metal of the faying surface to plasticize. The pin may be separately heated, but typically is designed to cause the metal to plasticize purely as a result of pressure without the need for additional heat or electricity. The pin moves along the length of the faying surface, and the plasticized metals from adjoining members are effectively “stirred” and intermix in the void created by the pin movement, thereby creating a weld seam. Additional filler material is typically unneeded. Because the yield strength threshold for various metals are usually well known, the FSW tool and pin can be precisely calibrated to apply no more than the exact pressure needed to cause the metal to plasticize and weld. This precise calibration also means that the weld joint cools and hardens almost immediately after the pin has passed a point in the faying surface. This results in a relatively instant weld without the application of broader heat, which can cause unwanted deformations.
In certain applications, the application of force from an FSW pin may be problematic. For example, where the desire is to join the abutments of horizontal panels in bridge decking, the application of a pin from above a panel, and the corresponding vertical pressure, can cause unwanted deformation in the decking panel. To solve this problem, developments have included the use of a pin with both an upper and lower shelf on either side of the welding probe. Rather than applying pressure perpendicularly to the panel surface, the FSW pin is instead applied to the abutment joint from a position parallel to the seam (i.e., from the side of the panel). Through this orientation, force perpendicular to the plane of panel is removed. Said dual-shoulder arrangement is described, for example, in U.S. Patent Publication No. 2014/0119814 A1. However, having a second shoulder on the distal end of a welding pin means that abutments must be located in flanged areas of the decking free from any obstructions intersecting with the flanged panels. This limitation means that the weld area is located in a region of the decking that is less stable as compared to other possible locations. For example, if it is possible for the faying surface to be in a location of the decking at, or close to the intersection of internal deck webbing members, the welds would be subject to a smaller torque moment, because the shorter span to transfer vertical load to the webbing members.
Aluminum decking panels are typically extruded or rolled. Although current methods for manufacturing aluminum decking are relatively precise, inconsistencies persist nonetheless. For example, aluminum panels may vary somewhat in thickness or extend away from webbing intersections at slight angles. Thus, when the decking members are intended to be joined at the abutments, these inconsistencies can make welding difficult.
Additionally, traditional FSW techniques impart not only axial forces normal to the plane of the abutment flanges, but FSW effectively imparts lateral forces as well. When an FSW pin imparts pressure on the panel abutments, each of the members naturally bias toward spreading apart. Accordingly, a successful FSW joint requires the user to precisely align the decking members and prevent them from spreading apart. However, given the typical arrangement of blunt faying surfaces, the addition of lateral alignment pressure can force the faying surfaces to shift vertically or laterally relative to the other, thereby creating the potential for a mismatched seam.
Accordingly, a need exists for a structure and method for joining bridge decking with FSW techniques without the need for manual alignment and without risk of causing mismatched seams.